This invention relates to transition metal catalysts and methods for their production and use in the polymerization of olefins.
In the production of polyolefins, such as for example polyethylene, polypropylene, ethylene-butene copolymers, polymethylpentene, etc., an important aspect of the various processes and catalysts used to produce such polymers is the productivity. By productivity is meant the amount or yield of solid polymer that is obtained by employing a given quantity of catalyst. If the productivity is high enough, then the amount of catalyst residues contained in the polymer is low enough that the presence of the catalyst residues does not significantly affect the properties of the polymer and the polymer does not require additional processing to remove the catalyst residues. As those skilled in the art are aware, removal of catalyst residues from polymer is an expensive process and it is very desirable to employ a catalyst which provides sufficient productivity so that catalyst residue removal is not necessary. High productivities are also desirable in order to minimize catalyst costs.
In the polymerization of propylene and higher olefins, it is desirable to produce isotactic polymer. Atactic polymer and low molecular weight polymer are soluble in the olefin monomer. High levels of the undesirable soluble polymer make operation of the polymerization reactor difficult if not impossible, especially for a continuous operation. Therefore, control of the amount of soluble polymer produced during polymerization is essential to a commercially viable process.
Soluble polymer production in high productivity catalysts is a function of both catalyst preparation and polymerization conditions. The changes in polymerization conditions, to decrease soluble polymer formation, include either the addition of external modifiers to the polymerization cocatalyst system, or the reduction of polymerization temperature. Reducing the polymerization temperature has the adverse effect of reducing catalyst productivity.
However, it would be desirable to develop new and improved catalysts and polymerization processes which provide relatively high catalyst productivities and lower soluble polymer levels.